In the operation of an internal combustion engine, the operator may have difficulty in starting the engine or in obtaining proper performance once the engine has been started. Such problems are typically due either to fuel system or electrical ignition system malfunctions. The operator must first determine which category of engine problem is most probably the cause of the malfunction, so that repair or adjustment efforts may be concentrated in the proper direction.
Efforts have been made to solve this problem by providing the operator with information relating to the performance of the electrical ignition system. U.S. Pat. No. 2,278,084 issued to T. L. Mayeux and H. A. Levey describes a transparent distributor cap which provides integral sensors for each spark plug cable. A hand-held device utilizing a transformer and glow lamp is described by W. J. Cook in U.S. Pat. No. 3,452,270. Spark plugs with internal light-emitting sensors are described by A. Candelise and J. A. Whaley in U.S. Pat. No. 3,242,366 and by R. W. Smith in U.S. Pat. No. 3,348,087. A voltage monitor is described by M. E. Gerry in U.S. Pat. No. 3,839,671.
In regard to the prior art, it may be seen that a device for measuring voltage at a spark plug high voltage terminal is useful but insufficient for reliable detection of faulty spark plugs. The presence of voltage at the spark plug terminal does not indicate that the spark plug has actually fired. A device which measures spark plug current flow is a more useful indicator of spark plug performance. A general indication of spark plug current is not certain proof of proper spark plug operation however, since current will flow across the conductive carbon deposit on a fouled spark plug insulator. A simple series spark gap or neon glow lamp is not able to distinguish between these normal and abnormal current conditions, since either case causes the spark gap or neon glow lamp to break down and emit light. A current sensing device would be a much more reliable indicator of proper spark plug performance (as well as the rest of the ignition system) if it had the capability to distinguish between normal and abnormal spark plug currents. It will be shown that the present invention meets this important requirement and has other new features which are significant improvements in this field of art.
The present invention provides a composite current sensor which has been designed to be selectively sensitive to the normal spark plug gap current and insensitive to abnormal currents which flow through conductive deposits on fouled spark plug insulators. The design is based upon observations of current waveforms present in spark plug high voltage leads in typical electric ignition systems. In normal ignition system operation, current does not flow through the spark plug until the high voltage source (usually an autotransformer) has charged the parasitic capacitance of the system to levels of several thousand volts. When the spark plug gaps arc, this parasitic capacitance acts as a relatively low impedance source of current. Peak normal spark plug current is an ignition system utilizing high resistance cable or resistor type spark plugs is on the order of 200 milliamperes. By contrast, current flow through a conductive deposit on the spark plug insulator is supplied primarily by the relatively high impedance high voltage source and is therefore lower in peak amplitude than the normal current. Tests indicate that peak abnormal current levels do not exceed about 40 milliamperes even when the spark plug insulators are short circuited to simulate very heavy conductive deposits. This information has been utilized in the design of the composite current sensor which is an important feature of the present invention. The composite current sensor is comprised of two basic elements which are connected electrically in parallel. The light emitting member of the composite current sensor is a two electrode gaseous discharge device, which may be either a neon glow lamp or a spark gap. The second member of the composite current sensor is a resistance element, which is connected in parallel with the gaseous discharge device. The composite current sensor is connected in series with a high voltage conductor, said high voltage conductor being used as a path for current between the ignition system high voltage source (typically an autotransformer) and a spark plug high voltage terminal. The parallel resistance element serves as a means to determine the sensitivity of the composite current sensor and thereby provide the sensor with the characteristic of being capable of distinguishing between normal and abnormal spark plug current. Before the correct value of resistance can be selected, it is necessary to know the breakdown voltage of the gaseous discharge device and the peak values of normal and abnormal spark plug current. For an example, consider the values of normal and abnormal peak current given previously for a typical automotive ignition system, i.e. 200 and 40 milliamperes respectively. If the gaseous discharge device is a typical high brightness neon glow lamp such as the NE-2H lamp, breakdown voltage will be approximately 100 volts. Prior to breakdown of the neon lamp, spark plug current will flow through the parallel resistance element. This resistance value must be smaller than a critical upper limit value which would develop the breakdown potential (100 volts) when the abnormal current was present (40 milliamperes). This condition sets an upper limit on the resistance value of 2500 ohms. In addition, the resistance value must be larger than a critical minimum value which just allows the breakdown potential (100 volts) to be developed in the presence of normal spark plug current (200 milliamperes). This condition sets a lower limit on the resistance value of 500 ohms. In summary, the resistance value is confined to a range between 500 and 2500 ohms so that the composite current sensor may be able to distinguish between the normal and abnormal spark plug currents. In practice, it has proven useful to select a value of resistance in the upper portion of this range, since peak normal spark plug currents are somewhat variable as the spark plug gap fires at slightly different potentials on each occasion. For this reason, values of approximately 2000 ohms have been successfully used in tests of the invention in simulated ignition systems and in actual automobile ignition systems. When dealing with an ignition system which does not use additional resistance (in the form of high resistance cables or resistance spark plugs) both the normal and abnormal spark plug currents are relatively higher, so that the appropriate value of parallel resistance in the composite current sensor will be lower. In practice, values of 100 to 400 ohms have proven to be appropriate in such ignition systems when used in combination with neon glow lamps with 100 volts breakdown potential. The use of spark gaps for the gaseous discharge device will typically require the use of relatively larger values of parallel resistance since breakdown voltages will be higher than 100 volts.
Tests of the present invention in simulated ignition systems as well as in typical automobile ignition systems indicate that the gaseous discharge device will emit sufficient light for viewing in typical daylight ambient light conditions when the ignition system is operating in a proper fashion. The tests show that the sensor will not emit visible light when the high voltage circuit is open, or when spark plug insulators are coated with a conductive carbon deposit. In the latter instance, carbon deposits were simulated by placing various resistors in parallel with the spark plug gap. The resistance value of these simulated carbon deposits ranged from 0.1 to 20,000 ohms and the sensor did not emit light under these conditions.
In addition to being able to distinguish between normal and abnormal currents, the present invention provides means for visual observation of the condition of the electrical ignition system at a location which is remote from the ignition system components. This is a particularly important feature for the operator of an automobile, where the operator must start and operate the engine at a location which is remote from the components of the ignition system. The present invention provides for location of the light emitted from the sensor at a position which is in optical communication with the operator of the engine. Two means are provided for this remote monitoring, which may be used separately or in combination. The first method involves the location of the gaseous discharge device at a position which is remote from the ignition system but within the visual field of the operator. This is done by providing a lengthened electrical conductor between the appropriate points on the ignition system and the terminals of the gaseous discharge device. Tests indicate that this method is quite effective, but electrical insulation around the extended electrical conductors must be able to withstand the high voltage on the ignition system without breakdown to nearby conductive bodies. This requirement leads to a somewhat bulky cable system when several spark plugs must be monitored. The second method involves the use of plastic or glass fiber optical light guides to transmit from the gaseous discharge device to a remote location which is in the field of view of the operator. In a typical instance, the receiving end of the fiber optical light guide would be in optical communication with the gaseous discharge device portion of the composite current sensor. The transmitting end of the fiber optical light guide would be placed in the instrument panel of an automobile or motorcycle for convenient viewing by the operator of the vehicle. In either of these two methods for locating the lights from the composite current sensor at a location remote from the ignition system, the present invention provides for functional enhancement of the use of the remote monitor by arrangement of the individual lights in a pattern which corresponds to the actual physical arrangement of relevant components in the ignition system. Typical patterns used as illustrative examples in the present invention include geometrical arrangements which correspond to the layout of spark plugs on the engine block or to the location of electrical connections on the distributor cap. In either case, a key would be provided which corresponded to some actual physical component on the engine or to a similar abstract symbol placed on the engine. This latter feature would serve the function of orienting the operator or mechanic in relation to the correspondence between the individual lights in the remote pattern and the actual components in the ignition system.
The invention may be provided as an add-on component for an existing ignition system, where the sensor units are added to existing high voltage conductors on the engine. As an alternate method of using the invention, the composite current sensors may be incorporated into the high voltage ignition cables at the time of manufacture of such cables. In the case of a single cylinder engine, such as is commonly used in motorcycles, lawnmowers and outboard motors for boats, only one composite current sensor unit may be installed in the high voltage connection between the ignition system high voltage source and the single spark plug. In the case of internal combustion engines with multiple spark plugs, one composite sensor unit may be in series with the high voltage cable which supplies current to each spark plug. While a discrete resistance element will probably be used as the parallel resistance element in the composite current sensor in the majority of cases, it is possible to utilize a section of high-resistance spark plug cable as a substite distributed resistance element to replace the discrete resistance element. In certain instances, this method may simplify the manufacture and cost of the composite current sensor provided by the present invention.